pathlib ABCs: support initializing paths with no arguments (#126608)

In the past I've equivocated about whether to require at least one argument
in the `PurePathBase` (and `PathBase`) initializer, and what the default
should be if we make it optional. I now have a local use case that has
persuaded me to make it optional and default to the empty string (a
`zipp.Path`-like class that treats relative and absolute paths similarly.)
Happily this brings the base class more in line with `PurePath` and `Path`.